Defibrillators are implanted in patients susceptible to cardiac tachyarrhythmias including tachycardia or fibrillation. Such devices provide cardioversion or defibrillation by delivering a high voltage shock to the patient's heart, typically about 500-750 V. Current devices typically apply this voltage between two transvenously placed electrodes: one at the distal end of a lead inserted into the patient's right ventricle (RV), and the other in the superior vena cava (SVC) region. A third electrode which is typically electrically coupled to the second electrode may be a subcutaneous patch implanted in the area of the left chest wall, or may simply be provided by the metallic housing or "can" of a pectorally implanted defibrillator. When the housing of the defibrillator is used as an electrode, it is sometimes referred to as an "active can" device.
Active can defibrillators enjoy the primary advantage that they do not require the additional surgical procedure to implant the subcutaneous patch. However, active can defibrillators used with RV and SVC electrodes may require a higher defibrillation threshold voltage than electrode systems using an RV, SVC and subcutaneous patch electrode combination. This may result from the longer conduction path between the heart and a pectorally implanted housing, as compared to the shorter path from the heart to a patch implanted beneath the skin of the left lateral chest wall.
An existing approach to improve the performance of active can devices is to increase the effective surface area of the can by attaching a sheet of mesh woven of conductive round wire to the housing. Such a device is disclosed in U.S. Pat. No. 5,385,574 to Hauser, et al. This may provide some reduction in the resistance of the interface between the housing and the surrounding tissue.
The disclosed embodiment provides an improved active can defibrillator by providing an implantable cardiac stimulator having a housing defining an interior cavity and having an exterior surface, and a defibrillation pulse generator circuit contained within the cavity. The housing exterior surface has a number of ridges. Each ridge may include at least two surfaces that are angularly offset from each other at an edge to form a sharp external corner. The ridges may be formed by creating grooves in the housing in a closely spaced arrangement.